Contrary to widespread opinion, the bloomery process CAN utilize
titaniferous ores, and indeed copes quite well with ores containing much
more than the 6.5% TiO2 in the ore cited by James Brothers. For
documentation see the following:
N.J. van der Merwe and D.J. Killick, "Square: an iron smelting site near
Phalaborwa" in N.J. van der Merwe and T.N. Huffman (eds.) Iron Age Studies
in Southern Africa, pp. 89-93. (Cape Town: South African Archaeological
Society, Goodwin Series 3, 1979).
E. Photos, H. Koukouli-Chrysanthaki and G. Gialoglou, "Iron metallurgy in
eastern Macedonia: a preliminary report" in B. G. Scott and H. Cleere
(eds.) The Crafts of the Blacksmith, pp. 113-120. (Belfast: Ulster Museum,
1984).
The South African case involves the smelting of rock ore
(magnetite-ilmenite bands in an ultrabasic PreCambrian complex), while the
Macedonian example involved the smelting of beach sands. I will confine
myself to the former, which which I am obviously more familiar. Some of
these magnetite-ilmenite ores run as much as 18% TiO2. How was it possible
to smelt these successfully in simple bloomery furnaces? The answer is
that because these furnaces were operated at much higher partial pressures
of oxygen than do blast furnaces, all of the titanium just goes straight
into the slag. Some of the bloomery slags we have analysed run 23% TiO2 !
I am extremely sceptical of claims that TiO2 makes bloomery slags too
viscous. My observations of the South African slags suggests that they
were fairly fluid - although they were not tapped, the exterior surfaces
have the smooth surfaces that are indicative of good fluidity, as do the
facts that there is little entrapped iron in them and that where they have
solidified around charcoal they preserve very fine detail of the charcoal
surfaces. I think that claims of this sort are inappropriate
extrapolations from blast furnace slags, which like in an entirely
different chemical system (CaO-MgO-SiO2-Al2O3) from bloomery slags
(FeO-SiO2-Al2O3). TiO2 does indeed make the former type of slag too sticky
- see in particular Turkdogan's book on metallurgical slags - but I don't
think that this is true of the iron-rich bloomery slags. The bloomery
furnace is a pretty versatile instrument - it can smelt ores that are way
outside the capability of modern blast furnaces.
----------------------
David J Killick
Associate Professor, Department of Anthropology
University of Arizona, Tucson, AZ 85721-0030
Phone (520)621-8685; FAX 621-2088
[log in to unmask]
http://www.mse.arizona.edu/faculty/killick.html
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